1. Field of the Invention
The present invention relates to a zoom lens. More specifically, the present invention relates to a zoom lens useful as a photographic optical system for a digital still camera.
2. Description of the Related Art
It is desired by the market that a photographic optical system used in an image pickup apparatus that uses a solid-state image sensor is a small-size zoom lens having a high zoom ratio. In order to implement a small-size camera using a zoom lens having a high zoom ratio, a conventional retractable type zoom lens is widely used that reduces the distance between lens units, when shooting is not executed, to a distance different from that during shooting and retracts the zoom lens into a camera body.
U.S. Patent Application Publication No. US 2008/0080062 and Japanese Patent Application Laid-Open No. 2004-102089 discuss an optical-path-bending type zoom lens that uses a prism having a reflection surface to bend an optical path of a photographic optical system, which is effective in reducing the thickness (the total front-to-back dimension) of a camera.
U.S. Pat. No. 7,426,085 discusses a hybrid type zoom lens between the retractable type zoom lens and the optical-path-bending type zoom lens, in which a lens unit located at a position closer to the object side than a reflection unit into a space that is generated by moving a reflection unit, such as a prism. Hereinbelow, the above-described hybrid type zoom lens is referred to as an “optical-path-bending retractable type zoom lens”.
In the zoom lens discussed in U.S. Patent Application Publication No. US 2008/0080062 and Japanese Patent Application Laid-Open No. 2004-102089, a second lens unit includes a reflection unit. Accordingly, the second lens unit does not move during zooming (for zooming). Furthermore, a first lens unit greatly moves during zooming. Thus, the zoom lens discussed in U.S. Patent Application Publication No. US 2008/0080062 and Japanese Patent Application Laid-Open No. 2004-102089, a second lens unit achieves a high zoom ratio. However, because the refractive power of the first lens unit is small, the moving amount of the lens units during zooming may become very great, which makes it difficult to reduce the thickness of the camera.
The optical-path-bending retractable type zoom lens discussed in U.S. Pat. No. 7,426,085 can reduce the thickness of the camera while achieving a high zoom ratio. However, because the optical-path-bending retractable type zoom lens discussed in U.S. Pat. No. 7,426,085 moves a lens unit located at a position closer to the object side than the reflection unit, it is necessary to design its optical system appropriate for the retraction of the reflection unit and the movement of the lens unit.
However, an optical system in which the first lens unit is greatly moved during zooming is not very mechanically useful. Furthermore, if an optical system like this is used, it becomes difficult to reduce the thickness of the camera while achieving a high zoom ratio.
More specifically, in order to retract a lens unit located at a position closer to the object side than the reflection unit while retracting the reflection unit, it becomes necessary to provide a large cutout to a cam tube for moving the lens unit located closer to the object side than the reflection unit to appropriately retract the reflection unit.
If the cutout like this is provided, it becomes difficult or impossible to secure a sufficiently large rotational angle of the cam tube. Accordingly, the first lens unit cannot be greatly moved during zooming.